There has been a need for an inexpensive, compact source of blue light for use in applications as diverse as flat displays for computers and entertainment systems, medical devices, analytical instruments, data storage, and optical communications. A semiconductor laser that could produce blue light would meet these requirements of low cost and small size. Semiconductor lasers have been used successfully to generate relative long-wavelength light such as green light (about 5,200 .ANG.), red light (about 6,500 .ANG.), or infrared (7,000 .ANG. to over 100,000 .ANG.). However, blue light has a relatively short wavelength (about 4,700 Angstroms) and lasers made of semiconductor materials have not given satisfactory results at such short wavelengths.
Some kinds of light-emitting diodes ("LEDs") are capable of emitting blue light. However, LEDs can only emit light over a relatively broad spectrum and therefore are not suitable for applications that require a single wavelength or a narrow range of wavelengths. In addition, the output power of LEDs is too low for many applications. Lasers can generate light at a single wavelength, but most lasers, especially semiconductor lasers, have not been able to generate light with a wavelength as short as that of blue light. For example, lasers fabricated of group III-V semiconductor compounds (usually compounds of gallium, indium or aluminum with arsenic or phosphorus such as AlGaAs or AlGaInP) do not produce significant energy--at room temperature--at wavelengths shorter than about 5,500 .ANG.. Other semiconductor light sources are very weak (e.g. silicon carbide), have a very short lifetime (e.g. polymers) or are too soft to work with easily and are not sufficiently reliable as yet (e.g. group II-VI compounds).
A semiconductor laser that can produce blue light was first reported by Hasse et al. in Applied Physics Letters, vol. 59, page 1272 (1991). This device is fabricated of a zinc-cadmium-sulfur-selenium compound and works at an extremely low temperature of 77.degree. K. (-196.degree. C.), not at room temperature. Continuous wave ("CW") devices that work at room temperature with reasonable lifetimes have not been reported.
Another approach to generating coherent short-wavelength light is through a second harmonic generation process in a nonlinear optical material. The second harmonic is twice the frequency, and hence half the wavelength, of the fundamental. For example, infrared light with a fundamental wavelength of 9,400 .ANG. has a second harmonic with a wavelength of 4,700 .ANG.. The intensity of the second harmonic is in general proportional to the square of the intensity of the fundamental, and if the available fundamental intensity is sufficient a strong second harmonic can be obtained. This approach has been used successfully with continuous-wave solid state lasers such as Nd:YAG. However, laser systems according to this approach have utilized multiple components and therefore have been physically bulky and expensive and have required very precise alignment. In addition, difficulties with stability and control have been encountered. Laser systems of this kind are described in such references as Yariv, Introduction to Optical Electronics (4th Ed.), Holt, Rinehart & Wilson, 1991; Yamamoto et al., IEEE Journal of Quantum Electronics, Vol. 28, page 1909 (1992); and Tamada et al., Proceedings of OSA Compact Blue-Green Lasers Topical Meeting, Santa Fe, N.M., page 120 (1992).
It is known to fabricate a monolithic device with a second harmonic generating element of a similar semiconductor material inside the cavity of a gallium arsenide or aluminum gallium arsenide edge-emitting semiconductor laser. However, these devices have long cavities which result in large absorption losses and phase matching difficulties. These disadvantages have made it impractical to generate useful second harmonic outputs with the desired wavelengths. Lasers of this kind are described in such references as Ogasawara et al., "Second Harmonic Generation in an AlGaAs Double-Heterostructure Laser, Japanese Journal of Applied Physics, Vol. 26, page 1386 (1987).
Monolithic surface-emitting devices having nonlinear regions have also been proposed. One such device, described by Vakhshoori et al., Applied Physics Letters, Vol. 59, page 896 (1991), has relatively low output power for the same reasons as the edge-emitting device. Moreover, the emitted light is distributed over a wide angular and spatial range and therefore is difficult to condense.
A more promising approach is described in copending U.S. patent application Ser. No. 08/047,969, filed Apr. 15, 1993 and assigned to the same assignee as the present application. This approach utilizes a vertical-cavity surface-emitting laser ("SEL"). The SEL is made of group III-V semiconductor material and is particularly fabricated to utilize certain nonlinear optical characteristics of the III-V material itself for frequency doubling purposes. This approach takes advantage of the relatively high intensity of light that is present inside the active region of an SEL. However, this approach requires a non-standard substrate orientation which may be difficult to manufacture, and the output power is limited by absorption of the frequency-doubled light in the semiconductor material.
Another approach is described in that certain U.S. patent application titled "Semiconductor Laser That Generates Second Harmonic Light With Attached Nonlinear Crystal" by Fouquet and Yamada, filed the same date as the within application and assigned to the same assignee as the within application, the contents of which are incorporated herein by reference.
Previous attempts to fabricate an SEL with an integral frequency doubling element have been constrained by a limited selection of suitable materials from which to fabricate a nonlinear crystal. The choices have been limited to materials having a crystal structure identical with that of gallium arsenide. It has been suggested that this limitation may be overcome by using wafer-bonding techniques to integrate a nonlinear crystal into a gallium arsenide SEL. This would permit the use of materials that have high nonlinear susceptibilities and low optical losses both at the fundamental frequency of the SEL and at the frequency-doubled second harmonic output.
Both reflectors in a practical SEL must have extremely high reflectivities. In addition, the SEL must be characterized by low optical losses, low parasitic resistance, and high thermal conductivity for heatsinking purposes. Many nonlinear dielectrics, among them LiNbO.sub.3, are not electrically conductive; this makes electrical injection difficult. In addition, one of the steps of fabricating the laser cavity is depositing one of the reflectors on the nonlinear crystal. This can be done by using dielectric distributed Bragg reflectors such as SiO.sub.2 or TiO.sub.2. However, these materials do not have a high enough thermal conductivity to provide adequate heat sinking. In addition, for long-term reliability it is necessary that the substrate, the nonlinear crystal and the reflectors all have similar thermal coefficients of expansion; this is not the case if materials such as those mentioned above are used.
It will be apparent from the foregoing that there is still a need for a compact, efficient semiconductor laser that can generate blue light.